1. Field of the Invention
The present invention relates to a filter circuit for use in a double-conversion TV tuner.
2. Description of the Related Art
To replace a conventional single-conversion TV tuner, there has been proposed a double conversion technique in which two-step frequency conversion is performed (in the case of a tuner used in Japan, an RF signal in a frequency range of 90 MHz to 770 MHz is first up-converted to a signal having a frequency of 965.25 MHz, then it is down-converted to a signal having a frequency of 58.5 MHz).
In contrast to the single-conversion technique which needs essentially tracking adjustment, this double-conversion technique does not need it. This prevents distortion in the output waveform which otherwise would occur due to the tracking adjustment error. Thus, the double-conversion technique is now receiving much attention as a next generation tuner technology.
A tuner using the double-conversion technique can be configured by using only one input filter for passing RF signals in a desired frequency band. However, in this case, all signals within the frequency band are applied to the following circuits including an RF amplifier, a first mixer, etc., and these signals produce intermodulation distortion. To reduce the intermodulation distortion, a conventional double-conversion tuner is usually configured such that the frequency band is divided into three to five narrower bands so as to separately receive the signals in respective divided bands. For example, in the case of a conventional double-conversion tuner shown in FIG. 3, there are provided three input filters having different pass-bands so as to separately receive the incoming signals for each of three frequency bands.
As shown in FIG. 3, from RF signals provided via an input terminal 1 which is one end of a signal transmission line, only signals within a desired frequency band are extracted in a filter circuit 2, and at the same time the levels of these signals are made at a predetermined level. Then, these signals are applied as an RF extracted signal to an RF amplifier 5. The RF extracted signal is amplified by the RF amplifier 5, then the signal is mixed with a first local oscillation signal output from a first local oscillator 6 at a first mixer 7 so as to up-convert the signal to a first intermediate frequency signal (965.25 MHz).
After the first intermediate frequency signal passes through a bandpass filter (BPF) 8, it is amplified by a first intermediate frequency amplifier 9 and applied to a second mixer 11. The first intermediate frequency signal applied to the second mixer is mixed with a second local oscillation signal output from a second local oscillator 10 so as to down-convert it to a second intermediate signal (58.25 MHz).
Then, after passing through a BPF 12, the second intermediate signal is amplified by a second intermediate amplifier 13 and output from an output terminal 14.
In the filter circuit 2 shown in FIG. 3, reference numeral 3 denotes BPFs 3 which allow RF signals having a frequency in the range of respective different frequency band to pass through them. Reference numerals 2a and 2b denote selection switches which exclusively select only one of BPFs 3 to which the RF signal is to be supplied, wherein selection-switching is performed according to selection signals SW1, SW2, and SW3 supplied from a control circuit (not shown) depending on a channel to be received. Reference numeral 4 denotes a variable attenuator for attenuating the RF extracted signal provided at the common terminal of the selection switch 2b to a predetermined level according to a gain control signal (hereinafter referred to as AGC voltage) supplied from a control circuit (not shown).
FIG. 4 shows an example of a circuit configuration for the filter circuit 2. In this figure, the elements corresponding to those in FIG. 3 are denoted by the same reference numerals and those will not be described hereinafter. In the filter circuit 2 shown in FIG. 4, although intermodulation distortion due to the large amplitude of an incoming signal does not occur, insertion loss is introduced by diodes such as switching diodes sd1 and sd4 for performing switching operation and a pin diode pd for attenuating the extracted RF signal which are inserted in a path through which RF signals and the extracted RF signal are transmitted.
FIG. 5 shows a filter circuit 2' which has been proposed to solve the above problem. In this figure, elements corresponding to those in FIG. 4 are denoted by the same reference numerals and those will not described again hereinafter. To reduce insertion loss due to the diodes, in the circuit shown in FIG. 5, the number of diodes inserted in the path through which the RF signals and the extracted RF signal are transmitted is reduced to two by arranging such that one diode acts as both of a pin diode and a switching diode. For example, in a path having a switching diode sd' and a pin diode pd1', no additional diode exists in this path and the total number of the inserted diodes in this path is two.
In the filter circuit 2', the attenuation associated with the extracted RF signal output from a selected one of BPFs 3' relative to the RF signal applied to the filter circuit 2' changes according to the AGC voltage. This change occurs in such a manner that the attenuation increases as the AGC voltage decreases and the attenuation becomes constant when the AGC voltage is lower than a predetermined value, as indicated by a curve 1 in FIG. 6, which is a typical characteristic of a reverse AGC type. The attenuation associated with the extracted RF signals output from the non-selected BPF3' is constant regardless of the AGC voltage as indicated by a curve 2 in FIG. 6.
More detailed description of the above technique can be found in Japanese laid-open. No. 3-39406.
Because the filter circuit 2' having a circuit configuration shown in FIG. 5 has such characteristics as described above, when the amplitude of the input signal is large, the difference in signal level becomes small between the extracted RF signal output from selected BPF 3' and the extracted RF signal output from non-selected BPF 3'. As a result, in this case, the extracted signal which is not extracted to a sufficient degree is applied to a following amplifier 5. This leads to a problem that intermodulation distortion easily occurs in circuits following the filter circuit 2'.